Continuing advances in miniaturizing electronic devices have resulted in an increased demand for rechargeable power sources that have long cycle life and can be rapidly charged. The life time of rechargeable batteries is limited to less than thousands of cycles due to the reduction-oxidation (redox) reactions which induce constant expansion-contraction of battery electrodes during charge/discharge cycles and consequently cause their deterioration over time. Also, with electrochemical energy stored in batteries delivered by the means of volumetric redox reactions, the charge/discharge rate and specific power of the best available lithium-ion batteries are limited by the slow rate of solid-state diffusion.
Electronic double-layer capacitors, commonly referred to as “supercapacitors”, are promising prospects for overcoming the life-time and power-density limitations of present rechargeable batteries. These supercapacitors store energy by nanoscale charge separation at the electrode-electrolyte interface. This charge storage mechanism involves no chemical reaction with the electrodes, and consequently results in fast charge/discharge rates in seconds while being able to withstand millions of charge/discharge cycles.
In order to obtain high energy density, supercapacitor electrodes are generally fabricated from electrically conductive materials having the high surface areas necessary for charge separation and storage at the electrode-electrolyte interface. Carbonaceous materials such as activated carbon are most commonly used. Recently, there has been interest in exploring carbon nanotubes (CNT) and graphene as electrode materials because of their high theoretical surface areas of 1320 m2/g and 2630 m2/g, respectively, and high electrical conductivity. CNT- and graphene-based supercapacitors have been demonstrated to possess high capacitance and fast charge/discharge characteristics. However, with regard to inkjet-printing these nanomaterials for practical applications, CNT and graphene suffer from: (1) high costs; (2) lack of commercial availability in large quantities, particularly for graphene; and, most importantly, (3) their tendency to aggregate in pure water due to their hydrophobic nature, even at very low concentrations of 5 ppm. The addition of surfactants can increase the stability of CNT and graphene dispersion, but most surfactants are found to have detrimental effects on capacitance.